In a satellite positioning system using a receiver of the GNSS type placed aboard a moving object, the data signals allowing the receiver to calculate its positioning originate from various satellites belonging to a constellation of satellites. The constellation comprises at least four satellites for determining four unknowns corresponding to the geographical x, y, z and temporal t coordinates of the receiver. The positioning of the moving object by the receiver is carried out in two steps. In a first step, the receiver effects the acquisition of radioelectric signals constituting navigation signals originating from the four satellites of the constellation and in a second step, the receiver evaluates the distances separating the moving object from the four satellites whose signals have been received and determines the position of the moving object by using a triangulation method.
An error made in the position of a moving object can have disastrous consequences in an application relating to civil aviation or geo-located road tolls.
There exist numerous sources of positioning error that may impair the validity of the position information determined by a satellite positioning system. A positioning error may be due to a technical problem with the reception of the GNSS signals, such as for example a failure of the receiver or a failure of the information transmitted by the constellation of satellites used. The reliability of the position determined by a satellite positioning system also depends on the environment in which the moving object is situated.
In the case of an aeronautical application relating to civil aviation, the receiver is not constrained by any obstacle, so that the radioelectric signals are received directly from the satellites, without reflection on any wall. In this case, there exist SBAS systems (Satellite-Based Augmentation Systems) making it possible to provide a confidence information item relating to the position calculated by the receiver of an aeronautical moving object. The SBAS systems monitor and bound, permanently, the errors made in the orbit of the satellites, in the synchronization of each satellite with the time reference of the constellations and the errors induced by the propagation of the radioelectric signals in the upper atmosphere and in particular in the ionosphere. The information provided by an SBAS system allows the receiver of the aeronautical moving object to provide the position of the moving object as well as a position error bound.
Geo-located road toll applications consist in determining the road followed by a terrestrial moving object furnished with a GNSS receiver and in billing a user of the terrestrial moving object when the road followed is subject to a toll. Billing being dependent on the road used, the receiver must deliver two complementary information items relating on the one hand, to the position of the moving object and on the other hand, to the trajectory of the moving object. These information items giving rise to billing, it is also necessary to determine a confidence information item relating to the trajectory used.
However, in the case of an application relating to geo-located road tolls, the conditions of reception of the radioelectric signals are much more complex, and much less controlled than in the case of an aeronautical application. It is then much more difficult to bound the position error determined by the receiver.
In an urban setting, the navigation signals emitted by one or by two or three of the satellites of the constellation may for example be stopped by buildings and not arrive at the receiver of the moving object. In this case, the geometry of the set of satellites that are used to calculate the position of the moving object is affected and this may render the calculation of the position of the moving object impossible.
Likewise, in an unfavorable terrestrial setting, the navigation signals emitted by a satellite of the constellation may be reflected on certain walls before reaching the receiver. This phenomenon, called multi-path, has a significant impact on the precision of the position calculated by the receiver. Indeed, the route measured by the receiver is then longer than the distance separating the moving object from the corresponding satellite. This results in an error in the triangulation method and therefore in the position of the moving object. In this case the consequence is twofold since on the one hand, the position error is significant and on the other hand, the receiver has no means of knowing that it has made an error, nor of evaluating the error made. Now, the errors made by the receiver may induce an error of judgment as regards the road followed and consequently induce a false billing. It is therefore necessary to obtain a confidence indicator relating to the trajectory of the moving object determined by the receiver.